Device and method for measuring capacitance and device for determing the level of a liquid using one such device

ABSTRACT

The invention relates to a device ( 5 ) for measuring capacity, said device comprising an electrode arrangement consisting of a plurality of electrodes (E 1 , E 2 , . . . , En) which are adjacently and/or successively arranged on a carrier ( 6 ), an intrinsic measuring device ( 8 ) for measuring the capacitance between a first electrode (E 2 ), in the form of a measuring electrode, and a second electrode (E 1 ), in the form of a counter-electrode, and a controllable switching deice ( 7 ) for connecting the electrodes (E 1 , E 2 , . . . , En), in the form of first and second electrodes (E 2 , E 1 ), to the measuring device ( 8 ) in such a way that they can be switched in a pre-determinable manner. The inventive device is characterised in that each electrode (E 1 , E 2 , . . . , En) of the electrode arrangement can be switched in a controlled, alternate manner by the switching device ( 7 ), in the form of a measuring electrode, and respectively at least one of the other electrodes (E 1 , E 2 , . . . , En), in the form of a counter-electrode, can be switched to a pre-determinable reference potential. The invention also relates to an associated method, and a device ( 1 ) for determining the level ( 2 ) of a liquid ( 3 ) using one such device ( 5 ).

The invention relates to device for measuring capacitance as claimed inthe preamble of claim 1 and a corresponding method and a device fordetermining the level of a liquid using one such a device.

The prior art, for example the publication of TOTH, F. N. et al.: A newcapacitive precision liquid-level sensor, Digest 1996 Conference onPrecision Electromagnetic Measurements, Braunschweig, discloses genericdevices. Here elongated “guard” and reference potential (“E₀”)electrodes are located parallel to a series of successive measurementelectrodes adjacently on one side or both sides. The capacitance andthus ultimately the level are measured by measuring the capacitancebetween the individual measurement electrodes and the opposing elongatedreference potential electrode.

These devices necessitate high complexity of interconnection and thushigh production and installation costs. In addition, special precautionsmust be taken to achieve high resolution with low sensitivity to noisesignals.

The object of the invention is to provide a device which overcomes thedisadvantages of the prior art. Moreover, the pertinent process and acorresponding means for determining the level of a liquid in a containerare to be made available. The device as claimed in the invention is tobe especially economical to produce and install and is to becontinuously reliable in operation. Resolution and (in)sensitivity tonoise signals be further optimized.

This object is achieved by the device as specified in claim 1 and by theprocess defined in the subordinate claim and the means defined in alikewise subordinate claim for determination of the level. Specialversions of the invention are defined in the dependent claims.

The object is achieved in a device for capacitance measurement with anelectrode arrangement consisting of a plurality of electrodes which arelocated next to one another and/or in succession on a support, an actualmeasurement means for measuring the capacitance between a firstelectrode as the measurement electrode and a second electrode as thecounterelectrode, and a controllable switching means for connection ofthe electrodes as the first and second electrodes to the measurementmeans, which connection can be switched in a definable manner, in thatcontrolled by the switching means each electrode of the electrodearrangement can be switched in alternation as the measurement electrodeand at least one of the other electrodes can thereby be switched as thecounterelectrode to a definable reference potential.

By preference, the electrodes are located on a surface area, especiallyon a planar surface. In one special embodiment the electrodes arelocated next to one another with their longitudinal sides in a rectangleshape. The distance of the electrodes is preferably as small aspossible, especially less than half, and preferably less than ⅕ of thedistance from one electrode to the next. For many applications it isadvantageous to arrange the electrodes on a flexible support, forexample on a film of polymer plastic such as for example polyimide. Theuse of materials with a low temperature coefficient of their dielectricconstant, such as for example polypropylene, is especially advantageousfor the flexible support and/or for a tube which optionally encloses thesupport.

The electrode support is preferably fixed or pressed with the electrodesinto stable contact with the inner side of the tube. For example, theelectrode support can be clamped onto an internal tube which iselastically deformable and which is inserted into the tube. The internaltube can be filled, especially foamed, for mechanical stabilization.

The tube, on its side facing the liquid, can be provided at leastpartially, preferably over its entire surface area, with a coatingwhich, as a result of the material chosen for the coating, causes forexample a high beading effect of the liquid and therefore reduceswetting with the liquid, and/or prevents diffusion of the liquid intothe tube. These coatings may contain for example a polymer plastic andcan be applied to the tube by painting or by an immersion bath.

Preferably the electrodes are not only electrically insulated againstone another, but are also covered with an electrically insulating layeron the side facing away from the support. It is advantageous if theelectrodes together with the connecting printed conductors are appliedto the support in thin or thick film technology. Application can takeplace in a structured manner, for example by screen printing orstamping. Alternatively or in addition, application can also take placeover the entire surface area and then the surface layer can bestructured, for example using photolithographic structuring processes,such as are known for example from semiconductor technology or hybridmicrocircuit technology.

In one special embodiment the device has a connecting means forelectrical connection of other sensors and/or for connection to theswitching means. The connection of other sensors and/or of the switchingmeans takes place preferably detachably and/or if necessary sealedrelative to the surrounding liquids.

Sensors may be provided which do not require direct contact with theliquids, for example a temperature sensor; in this case the sensor canbe located within the tube, for example on the electrode support, andcan make direct electrical contact with the printed conductors presentthere.

Alternatively or in addition, sensors may be provided which are to bebrought into direct contact with the liquid, for example a viscositysensor; in this case the sensor must be located outside of the tube, andelectrical connection takes place by way of an electrical penetration inthe tube that is impervious to fluids, especially on its bottom.Preferably there is a detachable plug connection.

Other sensors can be for example sensors for humidity, pressure or thelike, or also an additional capacitive sensor with which a medium whichsurrounds the device is examined with respect to its dielectricconstant. By preference, the connecting printed conductors for theadditional sensors are also mounted on the support of the device.

Furthermore, there can also be at least parts of the controllableswitching means or also the measurement means on the support of thedevice. As a rule, it is also possible to use as the support for theelectrode arrangement the same substrate as is used for the switchingmeans and/or the measurement means. The degree of integration ofcomponents ultimately depends on the respective application as well asthe requirements for the size of the device for which there can be lowerand/or upper limits dictated by the function of the device.

In one special embodiment of the invention, the definable referencepotential is the ground potential of the measurement means. In this waythe capacitance values of the switched electrodes can be measuredespecially easily in terms of circuitry and at the same time with a highdegree of precision.

Preferably the so-called “charge-transfer” process is used for themeasurement means. Conventional values of the capacitance to bemeasured, for example when using the device as claimed in the inventionas a level sensor, are in the range of fractions of a pF to a fewhundred pF, but can also be greater or smaller depending on the medium,especially its dielectric constant, and/or the electrode surface areasand electrode distances.

Preferably all electrodes which are not switched as measurementelectrodes are switched to the reference potential, to the groundpotential in particular. In the case of a level sensor it is moreoveradvantageous to switch the liquid and/or at least one part of the wallof the container to this or another definable reference potential.

All electrodes preferably have an essentially identical contour andsurface area. Preferably all electrodes are arranged essentiallyequidistantly to one another and/or to the connecting lines. This yieldsnot only simplified production of the device, but the measuredcapacitance values and capacitance changes are also fundamentally of thesame order of magnitude. When switching through in alternation, it ismoreover advantageous that the reliability of the device is increaseddue to the fact that the electrodes are identical. Moreover, theelongated reference potential electrode with a large surface area whichis not necessary as claimed in the invention clearly reduces the surfacearea required by the electrode arrangement, or the electrodes can belarger with the same required surface area, whereby the measurementsensitivity and/or the measurement accuracy is increased.

To increase the measurement accuracy at a given overall size of thedevice, it is also possible for several electrodes which are preferablynot directly adjacent to be interconnected hard-wired into onerespective electrode group. Each electrode group is alternately switchedas a measurement electrode, and at least one of the other respectiveelectrode groups is switched as the counterelectrode to the definablereference potential by the switching device. This corresponds todividing the individual electrodes into different component segments.The hard-wired interconnection of the electrodes into the respectiveelectrode group takes place preferably at the location of the connectinglines, especially at the height of the pertinent electrode, so that thedemand for space is not increased either with respect to the connectinglines.

The invention also relates to a process for capacitance measurementusing the above described device. Preferably the switching means iscontrolled by a microprocessor according to a control program which isstored in the microprocessor itself or in a memory component.

Moreover, the invention relates to a means for determining the level ofa liquid in a container with the device described in the foregoing. Inan evaluation means downstream of the actual measurement means themeasured capacitance is thereby compared to the stored reference values.These reference values can be fixed and invariable, or reference valuescan be stored depending on the application, especially depending on therespective liquid, and optionally also depending on the signals of theother sensors, such as especially of the temperature. The storedreference values can furthermore be adapted according to a givenalgorithm to the current actual boundary conditions, such as for examplethe temperature or viscosity of the liquid.

Preferably the electrodes are arranged in succession on the support withsuch a means in the immersion direction. When the level is determined,the individual electrodes are initially classified into “immersed”, “notimmersed” and “partially immersed” in a first step using the storedreference values or fixed expected values. The result of thisclassification delivers discrete values, for example “0” for “notimmersed”, “1” for “partially immersed”, and “2” for “immersed”.

An interpolation step then takes place for determining the level in thearea of the partially immersed electrode. The accuracy attainable inthis second, more or less analogous determination step depends on theheight h of the individual electrodes in the immersion direction andalso on the characteristic of the capacitance over level.

Other advantages, features and details of the invention arise from thedependent claims and the following description, in which severalembodiments are described in detail with reference to the drawings. Eachof the features mentioned in the claims and in the specification may beessential for the invention singly or in any combination.

FIG. 1 shows in a simplified representation a means as claimed in theinvention,

FIG. 2 shows in an enlargement the arrangement of the electrodes,

FIG. 3 shows the characteristic of the measured capacitance againstground over level,

FIG. 4 shows in an enlargement the lower end of the support,

FIG. 5 shows one alternative embodiment of the electrode arrangement,

FIG. 6 shows the characteristic of the measured capacitance againstground over level for the embodiment of FIG. 5.

FIG. 1 shows in a simplified representation a means 1 as claimed in theinvention for determining the level 2 of a liquid 3 in a container 4with the device 5 as claimed in the invention for measuring thecapacitance with an electrode arrangement consisting of a plurality ofelectrodes E1 to En which are arranged in succession on a support 6. Thedevice 5 furthermore has its own measurement means 8 for measurement ofthe capacitance between the first electrode E2 as the measurementelectrode and the second electrode E1 as the counterelectrode.Furthermore the device 5 has a controllable switching means 7 forconnection of the electrodes E1 to En as the first and second electrodesE2; E1 to the measurement means, which connection can be switched in adefinable manner.

The means 1 for determining the level 2 of liquid 3 furthermorecomprises an evaluation means 9 which is downstream of the measurementmeans 8 and which determines the level 2 from the capacitance measuredby the device 5 by comparison to stored reference values. This level 2can be output and relayed from the evaluation means 9 in optional,different ways, for example using a digital display 10, voice output ora warning signal 11 by means of a speaker 12, or for further processingto a control unit 13.

The controllable switching means 7, the measurement means 8 and theevaluation means 9 are preferably integrated in a microcontroller ormicroprocessor, especially in a single semiconductor chip, including amemory for reference capacitance values and for the control program.

FIG. 2 shows in an enlargement the arrangement of the electrodes E1 toEn; for reasons of clarity the support 6 is not shown. All electrodes E1to En are arranged in a rectangular shape and parallel to theirlongitudinal sides in succession on the support 6. The lower edges ofthe electrodes E1 to En are marked with level heights h1 to hn. Thedistance of any two electrodes E1 to En is a constant h. The connectinglines L1 to Ln to the individual electrodes E1 to En are routed up tothe measurement electronics, first in particular to the switching means7. Other connecting lines 14 run parallel thereto; other sensors locatedon the support 6 can make contact with them, for example a temperaturesensor 15 on the bottom end in the vicinity of the lowermost electrodeE1.

In one preferred embodiment, the electrodes E1 to En and the connectinglines L1 to Ln are attached to a so-called flex conductor film, i.e., toa very flexible thin substrate. The flex conductor film is located in anelectrically insulating tube which preferably consists of a materialwith a dielectric constant with a low temperature coefficient, such asfor example polypropylene.

The measurement means 8 determines the capacitance between therespective first electrode E2 which is used as the measurementelectrode, and at least one other electrode E1 which is positioned atthe ground potential of the measurement means 8. In one specialembodiment all the other electrodes which are not switched as ameasurement electrode are switched to ground potential by the switchingmeans 7.

Preferably however the electrode which is adjacent to the firstelectrode E2, especially adjacent underneath, is switched as the secondelectrode E1. Furthermore, the liquid 3 and/or, at any rate, one wall ofthe container 4 is also connected to the reference potential, especiallyconnected to ground.

The capacitance of the electrodes E1 to E5 which are completely orpartially immersed and which are located below the level 2 for liquidswith a dielectric constant of more than one is greater at any rate thanthe capacitance of the electrodes E6 to En which are located above thelevel 2. The level 2 is determined from the measured capacitances.

The determination of the level 2 takes place in two stages: first theelectrodes E1 to E4 are classified into “immersed”, E6 to En into “notimmersed”, and E5 into “partially immersed”. Then, if necessary,interpolation can be done using the capacitance value which is measuredfor the electrode E5 so that the exact level can be determined in thearea of the partially immersed electrode E5.

FIG. 3 shows the characteristic of the measured capacitance againstground over level. The difference of the capacitance value between the“not immersed” and “immersed” state of the electrode E1 to E5 in thisembodiment is approximately 2 pF at a base capacitance of approximately150 pF. In addition to the geometrical electrode arrangement, thiscapacitance difference is of course dependent mainly on the dielectricconstant of the liquid and accordingly in polar liquids such as water itis greater than in essentially nonpolar liquids such as oil. Thecharacteristic of the change of capacitance in all electrodes due to thesymmetrical arrangement is essentially identical and is marked by analmost linear average ascent, the start and end of which are rounded asa result of edge effects.

FIG. 4 shows in an enlargement the lower end of the support 6, which inthis embodiment is made as a flex conductor film which is placed in anelectrically insulating tube 16. On the lower closed end the tube 16 hasan electrical plug connection 17 for electrical connection of othersensors, for example a viscosity sensor.

To increase the measurement accuracy at a given total length of thelevel sensor, the height h of the electrodes must be reduced. The numberof electrodes would thus be increased, by which the number of signallines L1 to Ln and also the interconnection cost would be increased.

FIG. 5 shows one alternative embodiment of the electrode arrangement ofthe device as claimed in the invention. Here five individual electrodesE1 to E5 are divided into two component segments E1′, E1″, . . . to E5′,E5″. In this way the capacitance is increased between the respectivemeasurement electrode and the ground potential in several componentstages, in this embodiment in two respective component stages.Interpolation in the second step of signal evaluation thus becomes moreaccurate.

The illustrated embodiment shows a total of five electrodes which aredivided into two segments of the same size. Any other division isconceivable, for example also four electrodes into three respectivecomponent segments, six electrodes into four respective componentsegments, etc. The connecting lines of the respective component segmentsare connected to one another hard-wired directly on the support 6.

In this embodiment, the electrode E1 can still make contact with the twocomponent segments E1′, E1″ using a single connecting line 11. Thecomponent segments E1′ and E1″ of the first electrode are interconnectedhard-wired to form a first electrode group. This hard-wiring of theelectrode groups, of which there are a total of five in the embodiment,takes place preferably both with respect to the number of electrodescombined in one group and also with respect to the relative position ofthe electrodes combined in one group relative to the overall electrodearrangement such that the assignment of the measured capacitance value,which is to be undertaken by the means 1 for determining the level 2, toa resulting level 2 is well-defined, ambiguities in particular areavoided.

FIG. 6 shows the characteristic of the measured capacitance againstground over level for the embodiment of FIG. 5. Observation of thecurrent increase of the capacitance of an individual electrode E1 to E5generally does not yield unambiguous information about the number ofimmersed component segments E1′ to E5″. It is therefore advantageous tofirst undertake classification into “immersed”, “partially immersed”,and “not immersed” for all electrodes E1 to En. This takes placepreferably in that the capacitance values for “not immersed” are knownor have been determined beforehand and stored. After classification ofall electrodes has taken place, unambiguous assignment of the measuredcapacitance values to a level 2 is possible.

1. Device (5) for measuring capacitance with an electrode arrangementconsisting of a plurality of electrodes (E1, E2, . . . , En) which arelocated next to one another and/or in succession on a support (6), theactual measurement means (8) for measuring the capacitance between afirst electrode (E2) as the measurement electrode and a second electrode(E1) as the counterelectrode, and a controllable switching means (7) forconnection of the electrodes (E1, E2, . . . , En) as the first andsecond electrodes (E2, E1) to the measurement means (8), whichconnection can be switched in a definable manner, and controlled by theswitching means (7) each electrode (E1, E2, . . . , En) of the electrodearrangement can be switched in alternation as the measurement electrodeand at least one of the other electrodes (E1, E2, . . . , En) canthereby be switched as the counterelectrode to a definable referencepotential, characterized in that all electrodes (E1, E2, . . . , En)which are not switched as the measurement electrode are switched as thecounterelectrode, and that all electrodes (E1, E2, . . . , En) which areswitched as the counterelectrode are switched to the referencepotential.
 2. The device (5) as claimed in claim 1, wherein thedefinable reference potential is the ground potential of the measurementmeans (8).
 3. The device (5) as claimed in claim 1, wherein allelectrodes (E1, E2, . . . , En) have an essentially identical contourand surface area.
 4. The device (5) as claimed in claim 1, wherein allelectrodes (E1, E2, . . . , EN) are arranged essentially equidistantly.5. The device (5) as claimed in claim 1, wherein several electrodeswhich are preferably not directly adjacent (E1′, E1″; E2′, E2″; . . . ;En′, En″) are interconnected hard-wired into one respective electrodegroup, and wherein controlled by the switching device (7) each electrodegroup can be switched in alternation as the measurement electrode andthe other electrode groups can thereby be switched as thecounterelectrode to the reference potential.
 6. The device (5) asclaimed in claim 1, wherein the electrodes (E1, E2, . . . , En) togetherwith the connecting printed conductors (14) are applied to the support(6) in thin or thick film technology.
 7. The device (5) as claimed inclaim 1, wherein the device (5) has a connecting means (17) forconnection of other sensors (15) and/or for connection to the switchingmeans (7).
 8. The device (5) as claimed in claim 1, wherein thecontrollable switching means 7, the measurement means 8 and preferablyalso a downstream evaluation means 9 are integrated in a microcontrolleror microprocessor.
 9. Process for capacitance measurement with anelectrode arrangement consisting of a plurality of electrodes (E1, E2, .. . , En) which are located next to one another and/or in succession ona support (6), the actual measurement means (8) for measuring thecapacitance between a first electrode (E2) as the measurement electrodeand a second electrode (E1) as the counterelectrode, and a controllableswitching means (7) by means of which the electrodes (E1, E2, . . . ,En) are connected as the first and second electrodes (E2, E1) to themeasurement means (8) in a manner which can be switched in a definableway, controlled by the switching means (7) each electrode (E1, E2, . . ., En) of the electrode arrangement being switched in alternation as themeasurement electrode and at least one of the other electrodes (E1, E2,. . . , En) thereby being switched as the counterelectrode to adefinable reference potential, wherein all electrodes (E1, E2, . . . ,En) which are not switched as the measurement electrode are switched asthe counterelectrode, and wherein all electrodes (E1, E2, . . . , En)which are switched as the counterelectrode are switched to the referencepotential.
 10. The process as claimed in claim 9, wherein the switchingmeans is controlled by a microprocessor according to a stored controlprogram.
 11. Means (1) for determining the level (2) of a liquid (3) ina container (4) with a device (5) as claimed in claim 1 and anevaluation means (9) which is downstream of the actual measurement means(8) and which determines the level (2) from the capacitance measured bythe device (5) by comparison to stored reference values.
 12. The means(1) as claimed in claim 11, wherein the liquid (3) and/or at least partsof a wall of the container (4) are also switched to the referencepotential.
 13. The means (1) as claimed in claim 11, wherein in thedevice (5) for measuring capacitance several electrodes (E1′, E1″; . . .; E5′, E5″) which are preferably not directly adjacent areinterconnected hard-wired into one respective electrode group andwherein controlled by the switching device (7) each electrode group canbe switched in alternation as the measurement electrode and the otherelectrode groups can thereby be switched as the counterelectrode to thereference potential.
 14. The means (1) as claimed in claim 11, whereininterconnection of the electrode groups takes place both with respect tothe number of electrodes combined in one group and also with respect tothe relative position of the electrodes combined in one group relativeto the entire electrode arrangement such that the assignment of themeasured capacitance value, which is to be undertaken by the means (1)for determining the level (2), to a resulting level (2) is unambiguous.15. The means (1) as claimed in claim 11, wherein the electrodes arelocated on the inner side of a tube (16) which can be immersed into theliquid.
 16. The means (1) as claimed in claim 15, wherein the tube (16)on its side facing the liquid has a coating at least partially,preferably over the entire surface area.